I recommend that your student get a solid draft done the summer before senior year. It's such a crazy, busy year, it might be hard to find the time when school starts. Good writing requires revision, so it's important to have the time to do so.
While it's not a bad idea for underclassmen to get some thoughts on paper about a memorable experience, I find that kids can grow up a lot in a year. Since the goal is to present yourself as mature and independent, you don't want to finalize your essays too soon. For this same reason, students should not choose to retell an anecdote from childhood. Instead, they should focus on the last few years.
I can help students brainstorm, draft, revise, and perfect an essay that will showcase their talents and personality. The goal is to stand out, so the student should write in his or her own voice, use lots of detail, and let their creativity and humor shine through. Not that I can take the credit, but one of my students was just accepted to Princeton!